RU2393635C1 - Method of determining beat of optical fiber on transmission line section - Google Patents

Abstract

FIELD: physics.

SUBSTANCE: in compliance with proposed method, train of sounding optical pulses with duration exceeding pulse propagation time on beat length are sent into optical fiber transmission line closest end. Reverse Rayleigh process signal coming to closest end from optical fiber is sent to optical radiation polarisation analyser with its output receiving power of one-polarisation optical radiation. Reverse Rayleigh process characteristic is measured. Variable component of said characteristic is extracted to be divided into elementary sections to estimate mean beat length on every elementary section (LB)z as half the period of variable component variation along fiber. Proposed method differs from known designs in that duration of sounding pulse T₀ is varied linearly with preset spacing subject to the following conditions: v_g · (T₀) >> L_B and (T_{0 max} - T_{0 min}) << (T₀), wheree L_B is beat length, v_g is group speed of optical pulse propagation in fiber, (T₀) is mean duration of sounding pulse, and T_{0 max} and T_{0 min} are duration maximum and minimum values, respectively. Reverse Rayleigh process characteristic is repeatedly measured and memorised for every duration of sounding pulse. For every fixed coordinate z along fiber, beat length L_BT is defined as doubled period of variation of variable characteristic depending upon sounding pulse duration, esteems errors are defined as difference (L_B)₂-L_BT, beat length is defined equal to esteems of L_BT calculation of z at which error esteems do not exceed preset threshold.

EFFECT: reduced error of measuring beat length.

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Description

The invention relates to a fiber-optic communication technique and can be used to determine the distribution of the length of the beats of an optical fiber on a portion of a transmission line, which makes it possible to evaluate such characteristics of a linear path as the correlation length, polarization mode dispersion.

The methods of measuring the characteristics of the optical fiber / 1-3 /, based on the OTDR method, consist in the fact that an optical probe signal is introduced into the optical fiber at the proximal end, using continuous laser radiation modulated by a pseudo-random pulse sequence at the proximal end the optical radiation of backward Rayleigh scattering is received, the received signal is demodulated, and the desired character is determined from the obtained backscattering characteristic optical fiber optics. The permissible power of cw lasers introduced into the optical fiber is limited by the levels of laser breakdown. The cost of high-power continuous wave lasers operating in the operating range of quartz optical fibers is high. As a result, the length of sites controlled by the ROSE (Rayleigh Optical Scattering and Encoding) system that implements such methods is small in practice (about 5 km).

A known reflectometric method for localizing portions of a fiber optic transmission line with increased values of the polarization mode dispersion / 4 / is based on measuring the degree of depolarization DOP (Degree of polarization). This method does not allow to measure the length of the beats.

A known method of measuring the characteristics of the linear path of an optical transmission line / 5, 6 /, in particular the beat length of an optical fiber, is that at the proximal end of a fiber-optic transmission line, a sequence of optical probe pulses is introduced into the optical fiber with a duration of no more than half the time interval the propagation of an optical pulse in the area, the length of which is equal to the length of the beats, the backward Rayleigh scattering signal supplied to the proximal end from the optical fiber and the input optical radiation polarization analyzer, the output of which receive optical power and a polarization characteristic is measured backscatter, divide it into elementary portions and define the length of a beat on each elementary portion as a half-period change characteristics. This method for measuring the beat length of typical stepped single-mode optical fibers requires a probe pulse duration of less than 10 ... 20 ns. To measure the beat length of optical fibers with large values of polarization mode dispersion, the duration of the probe pulses should be even shorter. As you know, the pulse energy is proportional to its duration. As a result, with a short pulse duration, the dynamic range of the optical reflectometer is small. Accordingly, the requirements of this method for the duration of the probe pulses limit the dynamic range of the optical reflectometer and do not allow measuring the length of the beats on the transmission lines even of a relatively small length - 30 ... 50 km.

A known method for determining the length of the beats of an optical fiber in a section of a transmission line / 7 /, which consists in the fact that at the proximal end of a fiber-optic transmission line, a sequence of optical probe pulses is introduced into the optical fiber, the duration of which is several times longer than the propagation time of the pulses along the beat length, the Rayleigh backscattering signal arriving at the near end from the optical fiber is fed to the input of an optical radiation polarization analyzer, the output of which is power optical radiation of the same polarization, measure the backscattering characteristic, convert the measured backscattering characteristic so that distortions due to the shape and final duration of the probe pulses are suppressed, a periodic component is extracted, which is then divided into elementary sections, and the length of the beats in each elementary section is determined as half period of this component. It is known / 8 / that with an increase in the duration of the probe pulse, a dependence of the amplitude of the variable component of the backscattering signal on the duration of the probe pulse and the length of the beats of the fiber appears, and with the rectangular shape of the probe pulses, the amplitude of the variable component of the backscattering signal modulated with a period multiple of the length of the beats is proportional value:

where T ₀ is the duration of the probe pulse; ν _g is the group velocity; L _B is the length of the beats. Obviously, at 0.5 · ν _g · T ₀ / L _B ⇒n · π, where n = 1,2,3 ..., the amplitude of the variable component of the backscattering signal tends to zero and to isolate this variable component of the signal against the background of interference impossible. Moreover, the signal-to-noise ratio deteriorates even more with small values of the beat length. Given the random nature of the changes in the length of the beats, this leads to significant errors in the estimates of the average value of the length of the beats in the elementary section.

The essence of the invention is to reduce the measurement error of the length of the beats.

как половину периода изменений переменной составляющей характеристики вдоль волокна, отличается тем, что длительность зондирующего импульса Т₀ линейно изменяют с заданным шагом, обеспечивая при этом выполнение условий

и

, где L_B - длина биений, ν_g - групповая скорость распространения оптических импульсов в волокне,

- среднее значение длительности зондирующего импульса, а T_{0 max} и T_{0 min} - ее максимальное и минимальное значение соответственно, многократно измеряют и запоминают характеристику обратного рассеяния для каждого из значений длительности зондирующего импульса, для каждого фиксированного отсчета координаты z вдоль волокна определяют оценку длины биений L_BT как удвоенный период изменений переменной составляющей характеристики в зависимости от длительности зондирующего импульса, определяют погрешность оценок как разность

и определяют длину биений, принимая ее равной оценкам L_BT для отсчетов z, при которых оценки погрешности не превышают заданного порога.This essence is achieved in that a method for determining the beat length of an optical fiber in a portion of a transmission line, which consists in the fact that at the proximal end of a fiber optic transmission line, a sequence of optical probe pulses is introduced into the optical fiber, the duration of which is longer than its propagation time over the length of the beat The reverse Rayleigh scattering signal arriving at the proximal end from the optical fiber is fed to the input of an optical radiation polarization analyzer, at the output of which the power s optical radiation of one polarization is measured backscatter characteristic secrete variable component of the backscatter characteristics that is divided into elementary areas and determining an estimate of the mean value of beat length for each elementary portion

as half the period of changes of the variable component of the characteristic along the fiber, characterized in that the duration of the probe pulse T ₀ linearly change with a given step, while ensuring that the conditions

and

where L _B is the beat length, ν _g is the group propagation velocity of optical pulses in the fiber,

is the average value of the duration of the probe pulse, and T _{0 max} and T _{0 min} is its maximum and minimum value, respectively, repeatedly measure and remember the backscattering characteristic for each of the values of the duration of the probe pulse, for each fixed reference coordinate z along the fiber, an estimate of the beat length L _BT as the doubled period of changes in the variable component of the characteristic depending on the duration of the probe pulse, determine the error of estimates as the difference

and determine the length of the beats, taking it equal to the estimates L _BT for samples z, at which the error estimates do not exceed a given threshold.

The drawing shows a structural diagram of a device for implementing the proposed method.

The device comprises a probe pulse generator 1, the output of which is connected to the input of the optical radiation source 2 (laser), the output of which through the optical splitter 3 is connected at the near end of the transmission line to the optical fiber 4. At the near end of the transmission line, the optical fiber 4 is connected through the optical splitter 3 to the input of the polarization analyzer of optical radiation 5, the output of which is connected to the input of the photodetector 6. The output of the photodetector 6 is connected to the input of the processing unit 7, and the output of the processing unit 7 is connected with the input of the display unit 8. In this case, the second output of the probe pulse generator 1 is connected to the second input of the processing unit 7.

до

и удовлетворяет условиям

и

, где L_B - длина биений, ν_g - групповая скорость распространения оптических импульсов в волокне,

- среднее значение длительности зондирующего импульса, а T_{0 max} и T_{0 min} - ее максимальное и минимальное значение соответственно. От генератора зондирующих импульсов 1 эта последовательность зондирующих импульсов поступает на вход источника оптического излучения 2, с выхода которого оптические зондирующие импульсы через оптический разветвитель 3 поступают на ближнем конце лини передачи в оптическое волокно 4. На ближнем конце линии передачи сигнал обратного релеевского рассеяния из оптического волокна 4 через оптический разветвитель 3 поступает на вход анализатора поляризации оптического излучения 5, с выхода которого оптическое излучение обратного релеевского рассеяния одной поляризации поступает на вход фотоприемника 6, где преобразуется в электрический сигнал, который с выхода фотоприемника 6 поступает на вход блока обработки 7. При этом зондирующие импульсы со второго выхода генератора 1 поступают на второй вход блока обработки 7, обеспечивая синхронизацию, что позволяет измерить зависимость мощности обратного рассеяния от времени - характеристику обратного рассеяния. В блоке обработки 7 измеренные для каждого из значений зондирующего импульса характеристики обратного рассеяния запоминают. Для каждого фиксированного отсчета координаты z вдоль волокна определяют оценку длины биений L_BT как удвоенный период изменений переменной составляющей характеристики в зависимости от длительности зондирующего импульса, определяют погрешность оценок как разность

и определяют длину биений, принимая ее равной оценкам L_BT, для отсчетов z, при которых оценки погрешности не превышают заданного порога. Распределение длины биений по элементарным участкам выводится на дисплее устройства отображения 8.The device operates as follows. Generator 1 generates a sequence of probe pulses, the duration of which periodically with a given step varies linearly from

before

and satisfies the conditions

and

where L _B is the beat length, ν _g is the group propagation velocity of optical pulses in the fiber,

is the average value of the duration of the probe pulse, and T _{0 max} and T _{0 min} are its maximum and minimum values, respectively. From the probe pulse generator 1, this probe pulse sequence is fed to the input of the optical radiation source 2, from the output of which the probe optical pulses through the optical splitter 3 are supplied at the near end of the transmission line to the optical fiber 4. At the near end of the transmission line, the signal of the Rayleigh backscattering from the optical fiber 4 through an optical splitter 3 enters the input of the analyzer of polarization of optical radiation 5, the output of which is the optical radiation of the inverse relay scattering of one polarization is fed to the input of the photodetector 6, where it is converted into an electrical signal, which from the output of the photodetector 6 is fed to the input of the processing unit 7. In this case, the probe pulses from the second output of the generator 1 are fed to the second input of the processing unit 7, providing synchronization, which allows measurement backscatter power versus time - backscatter response. In the processing unit 7, the backscatter characteristics measured for each of the values of the probe pulse are stored. For each fixed reference, the z coordinate along the fiber determines the beat length estimate L _BT as the doubled period of changes in the variable component of the characteristic depending on the duration of the probe pulse, determine the estimation error as the difference

and determine the length of the beats, taking it equal to the estimates of L _BT , for samples z, at which the error estimates do not exceed a given threshold. The distribution of the length of the beats on the elementary sections is displayed on the display of the display device 8.

The length of the beats varies along the line. For a fixed value of the coordinate, the length of the beats, measured under identical conditions, takes a certain value. Accordingly, the oscillation period depending on the duration of the probe pulse can be determined with a significantly lower error than the oscillation period along the line length. Errors are also reduced by evaluating them by comparing the results obtained as a function of the duration of the probe pulse and by changing the backscattering characteristic along the line. Since the beat length varies along the fiber, the average value of the beat length over a certain section of the line is determined from changes in the characteristics of backscattering along the line. When determining the beat length, depending on the duration of the probe pulse, the beat length estimates are obtained for the coordinates along the line for the samples. As a result, the error in measuring the length of the beats is reduced and the resolution is improved.

LITERATURE

1. Patent US 2006/028636 A1.

2. Patent US 2006/028637 A1.

3. Patent US 2006/066839 A1.

4. Patent US 2003/174312 Al.

5. Patent WO 2005/041449 Al.

6. Galtarossa A., Menyuk C.R. Polarization mode dispersion. - Springer, 2005 .-- 296 p.

7. Patent RU 2325037 C2.

8. Jasenek J. The use of Polarization Optical Time-Domain Reflectometry for the birefringence distribution measurement along the SM optical fiber. - 12th International Scientific Conference "Radioelectronics 2002": Bratislava, Slovak Republic, 14. - 16.5, 2002, pp. 234-238.

Claims (1)

A method for determining the beat length of an optical fiber in a portion of a transmission line, which consists in the fact that at the proximal end of a fiber optic transmission line, a sequence of optical probe pulses is introduced into the optical fiber, the duration of which is longer than the time of its propagation over the length of the beat coming to the near end from optical fiber, the signal of the backward Rayleigh scattering is fed to the input of the analyzer of polarization of optical radiation, at the output of which the optical radiation power of one larization, measure the backscattering characteristic, isolate the variable component of this backscattering characteristic, which is divided into elementary sections and determine the average value of the length of the beats in each elementary section 〈L _B 〉 _Z as half the period of changes of the variable component of the characteristic along the fiber, characterized in that the duration of the probe pulse T ₀ linearly change with a given step, while ensuring that the conditions V _g · 〈T ₀ 〉>> L _B and (T _{0 max} -T _{0 min} ) <<〈 T ₀ 〉, where L _B is the length of the beats , V _g is the group propagation velocity of optical pulses in the fiber, 〈T ₀ 〉 is the average value of the duration of the probe pulse, and T _{0 max} and T _{0 min} its maximum and minimum values, respectively, measure and memorize the backscattering characteristic for each of the values of the duration of the probe pulse for each fixed reference coordinate z along the fiber beat length is determined estimate L _BT, as the double period variable component changes in characteristics depending on the length of the probing its pulse count error determined as the difference <L _{B> Z} -L _BT and determining a beat length, assuming it to be equal to L _BT estimates z counts at which the estimation error does not exceed a predetermined threshold.